The study of the regression of cancer can provide valuable information on the nature of the neoplastic process, the factors which control it and the relationship between the tumor and its host. We have developed a model system for the study of this process, which consists of a virus induced murine leukemia that spontaneously regresses. Our studies are focused on the virological, immunological, genetic and pathological factors involved in leukemogenesis and regression. We are also using this system to devise and test immunotherapeutic and chemotherapeutic regimens for the control and cure of leukemia. Our goals are the elucidation of the detailed mechanism of regression, the analysis of the role of various host factors in reversing this disease process, methods for enhancing these resistance factors, and, ultimately, the application of this information to human leukemias.